The present invention relates to an optical module for an objective. The invention can be used in connection with microlithography employed in the production of microelectronic circuits. It further concerns, therefore, an objective barrel which is suitable in particular for application in a microlithographic apparatus, as well as such a microlithographic apparatus including such an objective barrel.
In the field of microlithography in particular, it is necessary for the employed optical elements of the objective barrel, i.e. the lenses for example, to be positioned spatially with respect to one another with as high a degree of precision as possible, in order to achieve a suitably high image quality. The high precision requirements are not least a consequence of the constant demand to increase the resolution of optical systems used in the production of microelectronic circuits, in order to advance the miniaturisation of the microelectronic circuits to be produced.
With the increased resolution, the demands increase on the positioning precision of the employed optical elements. The latter must be maintained as far as possible in the installed state over the whole service life in order to avoid image errors. Furthermore, there is in this regard the requirement to achieve a dynamic behaviour of the employed optical system that is as favourable as possible, with resonant frequencies that are as high as possible.
For a large number of optical applications, but especially in the field of the aforementioned microlithography, objective barrels consisting of a plurality of optical modules are employed. The individual optical modules include as a rule an optical element, such as a lens etc., which is supported by means of one or more supporting devices at the inner circumference of a holder. Depending on the conditions of the optical system, i.e. amongst other things the optical properties of the objective that are to be achieved, it is often necessary to position several optical elements closely adjacent one another.
In the case of objectives with one lens per optical module, such as are known for example from EP 1 168 028 A1, a close arrangement of the lenses is achieved in which the supporting devices with the lenses located therein are arranged in a nested manner. This leads on the one hand to comparatively elongated lens barrels. This is due to the fact that the holder of each optical module must have a certain extension in the direction of the optical axis of the objective barrel in order to have sufficient strength and rigidity. Furthermore, the spacing requirement for the lenses together with the axial extension of the holder may give rise to very long supporting devices. These are disadvantageous particularly from the rigidity standpoint, since this is accompanied by an undesirably low rigidity and consequently undesirably low resonant frequencies.
There is proposed in document US 2002/0163741 A1 an arrangement of optical modules stacked one upon the other, wherein there is provided in the given holder a recess for the accommodation of a part of the supporting devices of the optical module lying thereunder. A reduction in the length of the supporting devices can certainly be achieved by this means. The recesses, however, in turn cause a weakening and reduction in rigidity of the given holder, and this has to be compensated for, possibly at high cost. On the other hand, this solution is suitable only for certain designs of the supporting devices.
The object of the present invention, therefore, is to make available an optical module of the type mentioned at the outset, which does not have the aforementioned drawbacks or at least only to a lesser extent and guarantees, in particular, a space-saving, rigid arrangement.